1. Field of the Invention
The invention is concerned with preserving the aesthetic appearance of architectural and aesthetic structures which are exposed to variable temperatures, moisture/drying cycles, freeze/thaw cycles, acidity variations, and the like, e.g. wood decks, buildings, freestanding walls, statuary, asphalt shingles, and the like. More specifically, the invention concerns the treatment of materials to prevent or remove disfiguring growths such as alga which darkens the materials.
2. Description of the Related Art
Many materials, particularly wood and cement building materials and the coatings thereon, may be subject both to freezing and extreme heat, or to repeated freeze/thaw cycles, which can embrittle, crack, or otherwise degrade the coatings. In other applications, building materials may be subject to high temperatures, as high as 130.degree. F. (54.degree. C.) on an asphalt shingle roof in the southern United States, and thermal degradation may result. Building materials are also exposed to ultraviolet light, which is well known to degrade many materials, and often subjected to repeated rinsing with water, perhaps pH as low as 4.0 ("acid rain"), and evaporative cooling. Building materials are subject to the abrasive action of wind and wind-borne particulates. Other contaminants, some of which may also be nutrients for algae, may also be present, such as calcium carbonate, dirt, oil, or plant material adhered to foundation blocks, and the like. Materials such as wood and stone may also be used indoors, and may be subject to varying temperatures and humidity, especially in bath areas. Since most buildings and recreational facilities have a life expectancy of 10 years or more, it is an object of this invention to provide long-term (greater than 10 years) protection from algae growth for such materials.
Concrete provides durable, inexpensive building facings, roofing tiles, and statuary, but can quickly develop dark streaks due to growth of algae which, initially airborne, lands on the structure and survives thereon. Other building materials such as wood, cement, concrete, stonework, masonry, and asphalt roofing shingles likewise can become streaked by algae. The discoloration of these materials is particularly noticeable when they have a light color.
The terms "algicidal" and "algae-resistant", when referring herein to chemicals or building materials means having the capability to kill or inhibit the growth of algae commonly associated with causing discoloration of roofs and other surfaces. McMahon (U.S. Pat. No. 3,507,676) identified the dominant organism causing such discolorization on ambient atmosphere exposed building materials as the alga Gloeocapsa magma. This particular alga is capable of being transported by wind currents, and it has been shown that calcium carbonate is a macronutrient for this alga, thus it is especially vigorous on calcium-carbonate containing surfaces such as marble. Alga are unicellular or polycellular plants, and are distinguished from fungi by the presence of chlorophyll and response to photosynthesis.
Alga may cohabit with fungi on building materials and cause further coloring problems. "Lichen" comprises algae and fungi which live symbiotically, i.e., two primitive plants, one with, one without, chlorophyll, which live together. Lichens yield coloring matter (litmus, orchil, zearin), acids (e.g., orsellic acid), carbohydrates, and depsides (esterlike anhydrides of phenolcarboxylic acids).
Limestone assumes a bewildering number of widely divergent physical forms, including marble, travertine, chalk, and the like, and contains from about 55 to about 95 weight percent calcium carbonate. Individual limestone types are further described by many common names, as detailed in Kirk & Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol..14, John Wiley & Sons (1981), pages 343-352.
Concrete, stonework, masonry, and wood can be coated with silicates or silicones (polysiloxane) coatings for water repellency, the latter such as disclosed in U.S. Pat. No. 4,877,654 (Wilson). Wilson describes aqueous emulsions useful (when applied and then cured into a polysiloxane coating) for rendering porous substrates water repellant comprising (a) a hydrolyzable silane having a predeterminable pH-stable range, (b) an emulsifying agent having an HLB value of from 2 to 20, an effective amount of (c) a buffering compound to maintain the composition within the predeterminable pH-stable range; and (d) water. It is also maintained by Wilson that such buffered compositions are stable on long term storage and maintain high effective levels of active silane content even when they include biocides which may accelerate the hydrolysis of aqueous silane-containing compositions.
Wilson is primarily concerned with water repellency of polysiloxane-coated substrates such as concrete suspended in distilled water for 21 days, as exemplified by the tests described in cols. 7 and 8, not with biocidal activity after prolonged exposure to varying ambient atmospheric conditions such as changing pH, temperature, and humidity conditions. In order to render the polysiloxane-type coatings described by Wilson resistant to unspecified organisms, Wilson notes that the uncured composition must be buffered to make the hydrolyzable silane monomers hydrolytically stable in the presence of pH shifting additives, such as biocides. However, Wilson does not disclose by way of any example that treatments with his emulsion composition (including a biocide) and subsequent polysiloxane coating were proven to render concrete resistant to algae or any other organism. Further, the coatings applied by Wilson are thick (1 liter/m.sup.2), and if the concrete structure is not allowed to "breathe", it is well known that concrete structures soon deteriorate. It is undesirable to water proof such materials since they tend to crumble if they cannot breathe; thicker coatings tend to be more water proof or water resistant, and thicker coatings tend to result in higher material costs.
German Offenlegungsschrift DE 3828775 (Huttinger) describes antimicrobial compounds for controlling undesirable growth of microbes such as E. coli in water-bearing devices, e.g., a device containing glass microbeads for disinfecting water. In one example, glass microbeads impregnated with or coated with 3-(triethoxysilane)-ethyl-tri-n-butylstannate are tested for their ability to kill E. coli. In a flow through experiment, no E. coli was killed. However, when water was kept in contact with treated beads for one day E. coli were killed.
Huttinger theorizes that since the tin compounds are completely apolar, adsorption of E. coli did not take place, so that none were killed, but after flow interruption, the E. Coli sedimentized and were killed upon contact with the tin-silane coated glass surface.
Huttinger also disclose paper treated with one of the antimicrobial compounds and this is indicated to be useful for filtering bacteria from air.
However, from the scant information provided by Huttinger, the coating thickness or integrity cannot be determined, and the treated glass is shown to be effective against E. coli only when a static water sample is kept in contact with the coated beads for one day. Although a different mode of action is proposed, there is no evidence given by Huttinger that the antimicrobial compounds are not simply leaching or desorbing, in an equilibrium reaction, from the from the glass substrate. Further, there is no demonstration of effectiveness against other, more robust organisms, or against organisms in the presence of a macronutrient for that organism., and it is specifically noted that the treated glass is not effective even against E. coli when the aqueous medium is flowing. It is highly speculative whether the microbial action against E. coli could be extended to more robust organisms, such as the blue-green algae Gloeocapsa magma, which is transported by air-borne particles and is able to tolerate a variety of climates due to the desiccation properties of the gelatinous sheath surrounding the algae cells. Alga colonies are not known to infest glass surfaces, such as used by Huttinger.
Therefore, although compounds useful in the present invention are described in Huttinger, there is no suggestion that the compounds would be effective against established or freshly introduced alga colonies on wood, concrete, and other non-glass building materials, especially in the presence of calcium carbonate, and there is no teaching of effectiveness beyond a period of a day or a few days.
It an object of the present invention to provide a method of rendering wood, concrete, and other non-glass materials algicidal by immobilizing an algicide thereon, rather than simply compounding a biocide into a polymeric coating, which is known to provide materials which readily leach the biocide.
Tin compounds can afford fungicidal wood protection as reported in Kizlink, "Tributyltin-N N-dialkyldithiocarbamates, as Fungicides for Wood Preservation Against Rot" JOCCA Vol 74 No 9 1991 pp 329-330. The Kizlink publication also cites prior publications concerning the application of tin compounds as corrosion inhibitors, fungicides for plastics, paper, and paper pulp, and as biocides. In spite of the general effectiveness of tin compounds, however, they are sufficiently soluble in water to leach, and tin compounds are potentially toxic. So, while tin and/or tin compounds were widely incorporated into marine paints and used in other applications, it is presently undesirable to randomly select a tin compound for use in these applications, in view of greater concern for the environment and human health. To be effective against alga colonies, the algae-resistant compound and the resulting coating or treatment must be resistant to thermal, hydrolytic and photochemical degradation, under conditions far more extreme than those encountered in laboratory water or marine environments.
Algae growth on ceramic-coated roofing granules of asphalt roofing shingles can be virtually eliminated by incorporating a copper compound, such as cuprous oxide, into the ceramic coating. Such techniques are disclosed in assignee's pending patent application Ser. No. 07/945,127, filed Sept. 15, 1992. However, the cuprous oxide loadings increase the cost of manufacturing the granules, and the copper compounds readily leach out of the coating, thus diminishing their long-term effect. If copper could be incorporated into concrete, it might likewise inhibit algae growth, but copper would interfere with the setting up of the concrete, and the method is not useful for existing structures.
Applicants' previously cross-referenced patent describes and claims roofing shingles and methods of protecting same against algae streaking, the shingles comprised of roofing granules incorporating an effective amount of a compound of the general formula EQU (R.sub.n Sn)--[R'--(SiX.sub.p R".sub.3-p) ].sub.4-n (I)
wherein
R is an organic radical of the formula C.sub.z H.sub.2z+1 wherein z ranges from 1 to 8, PA1 R' is a divalent radical of an aliphatic hydrocarbon containing 2 to 20 carbon atoms, PA1 R' is an organic radical containing 1 to 8 carbon atoms, PA1 X is a hydrolyzable group, PA1 n is an integer from 0 to 3, and PA1 p is an integer from 1 to 3. PA1 1) applying to a material to be protected or restored, and/or PA1 2) incorporating into a material, thus forming a composite, an effective amount of a compound containing tin covalently bonded to a silylating group such that cleavage of the covalent bonds due to hydrolysis or photolysis is minimized, thus making the treatment useful for long periods of time (at least 10 years). PA1 R is an organic radical of the formula C.sub.z H.sub.2z+1 wherein z ranges from 1 to 8, preferably 3 to 6, more preferably 4; PA1 R' is a divalent radical of an aliphatic hydrocarbon containing 2 to 20 carbon atoms, preferably 2 to 8 carbon atoms; PA1 X is a hydrolyzable group, preferably selected from the group consisting of halogen and alkoxy groups; PA1 n is an integer from 0 to 3, and PA1 p is an integer from 1 to 3.
Although this work is impressive, there is still a long-felt but unmet need in the art of non-particulate building materials, statuary, recreational facilities, and the like, which are exposed to changing ambient conditions (either indoors or outdoors) or other conditions conducive to algae growth, to restore or render them algae-resistant, while meeting increasing regulatory demands for low release of metals into the environment. The restoration and/or protection must be for long time periods, with little or no effects on water repellency, color, or other properties of building materials. In the absence of an adequate solution to this long-felt need, it still has been necessary to clean even water repellent concrete which is exposed to ambient environments periodically, usually with bleaches or phosphates, to remove algae growth. The cleaning operation is time-consuming and expensive and does nothing to inhibit recurring growth.